1. Technical Field
The present invention relates to rotating electric machine drive systems that are used in, for example, motor vehicles to drive electric motors and electric generators.
2. Description of Related Art
Conventionally, it has been desired for electric motors for electric vehicles to exert high-torque and high-efficiency characteristics over a wide operating range from low speed to high speed.
To satisfy such a desire, there has been developed a method of switching the connection of a stator coil of an electric motor between a Y connection (or star connection) and a Δ connection (or delta connection) so as to enable the electric motor to exert optimal characteristics in both a low-speed region and a high-speed region (see, for example, Japanese Patent Application Publication No. JP2014054094A).
It is required for four-wheel drive vehicles and heavy medium-sized or large-sized vehicles to have an excellent hill-climbing ability. Moreover, in an engine-starting operation, diesel engine vehicles generally require twice or more torque than gasoline engine vehicles.
The above-requirements may be met by increasing the amount of electric current supplied to rotating electric machines used in the vehicles or increasing the sizes of the rotating electric machines.
However, increasing the amount of electric current supplied to the rotating electric machines would result in increases in the copper and iron losses of the rotating electric machines. On the other hand, increasing the sizes of the rotating electric machines would make it difficult for the rotating electric machines to be received in a limited space in an engine compartment.